A problem in the reception of multi-level modulated signals is that the carrier wave is difficult to regenerate, because the modulated, received signal does not contain a real, unmodulated carrier wave. The effective information in a multi-level modulation, such as that of 64-QAM (Quadrature Amplitude Modulation) and 256-QAM, deforms the carrier wave phase at random to so many possible angles that in most prior art synchronization methods the effective information itself disturbs the estimation of the carrier wave phase (self-noise). The larger the employed modulation alphabet, the worse the problem generally becomes.
A prior art carrier wave synchronization method is based on the raising to a power of the received signal, as well as on filtration. Among the drawbacks of this method in the reception of multi-level modulated signals, is the fact that the carrier wave synchronization signal to be generated contains a strong insignificant part and weak useful part. The useful part describing the phase error is made available with a dense but slow filtration (a kind of averaging). If the frequency error between the local oscillator frequency and the carrier wave is significant, the phase error may change during the averaging to such an extent that the synchronizing signal is not kept up to date; it may exchange its sign, vibrate or be eliminated through averaging.
Another prior art carrier wave synchronization method is based on a process of detecting the transmitted data symbols from the received signal by applying certain decision-making criteria. Such methods are mainly designed for phase tracking after the receiver is locked to the carrier wave. An example of this kind of solution is DDL (Decision Directed feedback Loop). In carrier wave synchronization, this type of solutions based on decision-making can finction with extremely low modulation alphabets, such as BPSK (Bi Phase Shift Keying) and QPSK (Quadrature Phase Shift Keying). In order to ensure a sufficiently reliable operation, the error ratio in the decision-making is typically 1:100. In multilevel modulations an error ratio of this order means that the demodulated signal point system remains more or less inside the decision-making grid and is not allowed to rotate, as is illustrated in FIG. 1a. With multi-level modulation alphabets, such solutions are applicable mainly in the correction of the phase error in a situation where the frequency is corrected, in other words when the local oscillator frequency is locked to the carrier wave frequency.